Means for improving the sensitivity and the response characteristics of velocity microphones



1951 H. F. OLSON ET AL 2,566,093

MEANS FOR IMPROVING THE SENSITIVITY AND THE. RESPONSE CHARACTERISTICS OFVELOCITY MICROPHONES Filed June 50, 1948 up I INVENTORS Herr 7E Ulwon 60 12 Pr fan ATTORNEY Patented Aug. 28, 1951 MEANS FOR. IMPROVING THESENSITIVITY AND THE RESPONSE CHARACTERISTICS ,OF VELOCITY MICROPHONESHarry F. Olson, Princeton, and John Preston,

Metedeconk, N. J assignors to Radio Corporation of America, acorporation of Delaware Application June 30, 1948, Serial No. 36,196

UNITED STATES PATENT OFFICE 12 Claims.

This invention relates to electro-acoustical translating apparatus, andmore particularly to a microphone of the dynamic type.

Sensitivity and wide frequency range have always been two incompatiblefactors in electroacoustical translating apparatus, such as microphones,especially the type of apparatus used in radio broadcasting. With anextension of the frequency band of radio broadcasting, as in the case offrequency modulation and high quality amplitude modulation, noise in thesystem becomes a very important problem.

The-velocity type microphone, to which our present invention isdirected, is almost universally employed in broadcast practice, sincethe sensitivity is as great as, or greater than other types ofmicrophones in use at the present time.

Since the velocity type microphone is highly of the problem is to makethe microphone more w sensitive. While weight is not a factor, if it iskept within reason, restrictions on size must be considered. We haveprovided an improved ve-- locity microphone, which not only overcomesmany of the problems encountered by similar microphones in use at thepresent time, but which also has greater sensitivity and improvedresponse characteristics. Thus, the primary object of our presentinvention is to provide an improved velocity type microphone thesensitivity and the response characteristics of which are greatlyimproved over those of similar, prior art microphones, with nosubstantial increase in the size thereof.

It is also an object of our invention to provide an improved velocitytype microphone which will eliminate or reduce non-uniform response withrespect to frequency.

It is another object of our invention to provide an improved velocitytype microphone which will average out diffraction and resonance effectsand provide a smoother response frequency characteristic.

It is a further object of our invention to provide a high sensitivityvelocity microphone in which the difference in sound wave pressure onthe front and back of the ribbon diaphragm will continue to increaselinearly with the frequency and thereby prevent the falling off ofresponse in the high frequency range.

It is also an object of our invention to provide a high sensitivityvelocity type microphone which is simple in constructionand at the sametime highly efficient in use.

In accordance with our present invention, we have provided an improvedvelocity type microphone in which we make use of a pair of pole pieceseach of which has an intermediate portion of reduced cross section toshorten or reduce the ribbon diaphragm. An unsymmetrical magnetic fieldstructure is connected to the ends of the pole pieces in order toaverage out diffraction and resonance effects characteristic ofsymmetrical field structures. In addition thereto, the microphone isalso equipped with a high frequency equalizer of the izind disclosed in.our copending applioationSer. No. 29,844, filed May 28, 1948, formaintaining suitable response throughout the high frequency range.

The novel features of our invention, both as to its organization andmethod of operation, will best be understood from the following detaileddescription when read in connection with the accompanying drawing, inwhich Fig. 1 is a front elevation, partly cut away of a velocity typemicrophone constructed in accordance with our present invention,

Fig. 2 is a front elevation of the microphone field structure shown inFig. 1, with the high frequency equalizer plates removed,

Fig. 3 is a side view, in section, of the field structure illustrated inFig. 2, equipped with high frequency equalizer plates, and taken on theline a-a of Fig. 2,

Fig. 4 is a cross section of the pole pieces and ribbon diaphragm of ourimproved microphone, taken on the line 4-4 of Fig. 2,

Fig. 5 is a front elevation of a modification of a velocity microphonefield structure constructed in accordance with our invention, with thehigh frequency equalizer plates removed, and

Fig. 6 is a side view, in section, of the field structure illustrated inFig. 5 equipped with high frequency equalizer plates, and taken on theline 6-6 of Fig. 5.

theacoustic path'from the front to the back of Referring, now, in detailto the drawing in which the same reference numerals refer to likeelements, there is shown in Fig. l, a Velocity type microphone Iconstructed in accordance with our invention, The microphone l ispivotally mounted on a support 3 of conventional design, and comprises,essentially, a ribbon diaphragm 5, a pair of pole pieces 1, 9, and apair of magnets consisting of top and bottom units ll, [3.

According to one form of our invention, as shown in Figs. 1, 2, 3 and 4,the microphone i is provided with a pair of elongated pole pieces I, 9disposed in spaced, parallel relation to provide an air gap ,[5 betweenadjacent flat faces ll, IQ of the pole pieces. The pole pieces may be ofany desired shape. As shown in Fig. 4, the pole pieces illustrated aresubstantially circular in cross section with the exception that they areprovided with the fiat faces I], I9 disposed on adjacent sides of thepole pieces I, 9. Each of the pole pieces I, 9 has an intermediateportion, the cross sectional area of which decreases toward the centerthereof, forming longitudinal concave surfaces 25, 21 around the polepieces except on their adjacent sides, or flat faces ll, I9. The cut-outintermediate portion of the pole pieces I, 9 serves to reduce theacoustic path from the unsymmetrical field structure.

. 3 front to the back of the ribbon, whereby good response is maintainedthroughout the high frequency range.

The ribbon diaphragm 5 is suspended in the air gap i5 between the polepieces 7, 9, and is held in place by two insulated clamps '29, 31carried by the field structure. The ribbon 5 is also maintained inspaced relation to the pole pieces so that it will be free to vibrate inresponse to differences in sound wave pressures on the front and back ofthe ribbon.

The top and bottom unit il, 13 comprise two substantially 'U-s'hapedmagnetic structures which are attached opposite ends 33, 35, 3'5, 39 ofthe pole pieces l fiffor supplying magnetic fiux across the air gap ldin a manner well known in the art. Referring to Fig. 3 off thefdrawing,it will be seen that the bottom unit i3 'is larger than the top unit 'Il, thus forming an unsymmetrical field structureior the microphone. Forsimplicity of construction a-nd to facilitate manufacture, the units ii, is contain combinations of magnetizing bodies or permanent barmagnets each of equal size made from material'having ahigh degree ofretentlvi ty, such as Alnlico V (composed of aluminum 8 percent, nickeli'ipercent, cobalt 24 percent, copper 3 percent, and the remainderiron). As shownin Figs? and "'3 of the drawing, the top unit it hastiwob ar magnets d3 disposed in spaced, parallel relat'ionyw'ith theends :35, 47 of these bar magnets connected by a soft iron plate elementThe opposite ends 5%, "53 of the bar magnets are respedtively co'nnectedto outer pole rnezrib'ersfidtl which, in turn, are connected to the topends "33,37 of thepole pieces E, ii. The bottom un'it l 3'has two pairsof bar mag provided with a high frequency acoustic equalizer or" thetype described in our above-identified copending application. Theequalizer comprises two perforated plates or screens 85, 83 which aredisposed in spaced,parallel, overlapping relation to the ribbondiaphragm The equalizer plates 81, 83, may be supported in any suitablemanner to insulate them from the magnetic structure (for example, byattaching them to the insulated ribbon clamps 29, Each of the plates hasa plurality of perforations 85. The size of the perforations or openings85, as well asthe spacing between the plates 81, as "and the ribbon -5are determined according to the frequency range to be compensatedfor,'and'inay be varied according to the condition to befc'orrect'ed.The plates -8i, 83 form resonance chambers on opposite sides of theribbon '5 and operate as reflectors for sound Waves transmitted throughthe perforations whereby a standing Wave system is produced between theplates which will maintain the necessarydifference in sound wavepressure on the front and back of the ribbon and prevent the falling offof response in the high frequency range. 7

Referring to Figs. 5 and 6; there is shown a modified form of fieldstructure 9 i for a velocity type microphone constructed in accordancewith our invention. The microphone itself comprises the sameessaitial'elementsas that-shown in'Figs. 1 through 3 and differsprimarily in respect to the field structure. The modified fieldstructure t! comprises, essentially, a pair of pole pieces-9%,

nets 53, $5, each of which is the same as the bar to the'lower ends35,1339 ol the polepieces l, 9. g

The plate elements'an dthe outer pole members should preferably be-inadefrom analloy or combination of alloys Which'is capable if carrying ahigh flux density before becoming saturated.

(For'examplefwe 'have found that an alloy con= taining 50 percentcobalt, and "5!) -,percent iron will carry more flux than pureiro'n issatisfactory for the field structure of our .pres'ent'invention.)

As pointedout above, the use oftwo barmagnets M, 53 in'the topurii't Hand'tw'o pairs'of bar.

magnets d3, 85in the bottom'unitl'ii defines an By providin anunsymmetrical fie1dstiucture,tWo advantages result. (l) Thefield's'tructureforms two cavities '78, 8d of different dimensionsbe'tween'the bar magnets the plate elements and theends ofthe polepieces. Under these conditions a fundamental resonance at diiferentfrequencies is obtained which averages out the deleterious eiiects ofthese resonances upon the response. '(2) "Providing anunsymm-etricalfield structure also varies the path length betweenopposite sides of the'ribbon diaphragm, as a result of which, thenecessary difference in pressure, due to the diflerence in phase, isalso varied. 'These difierentpath lengths extend an definite cut-offfrequency in the re sponse, which, under symmetrical conditions wouldoccur.

Our improved velocity microphone l is also ill, and top and'bottom'magnet units 98, it i. The pole pieces are spaced apart toprovide an air gap in which adiaphra nrdsisreceived. The top magnet unit"9 comprises twoicylindrical-b'ar magnets its, 35 which aredisposed inspaced, parallel relation so-as-to provide-an airspace Mil between them.The bar magnets its, we have two outer pole-elements'or members 1%, i Hrespectively connecting the endsof the magnets to the upper ends H3, iof the pole pieces-95,97 to provide a substantially U 'shaped magneticstructure. The bottom magnet unit til is similar to the top unit-ddexcept-that itha's three bar magnets HT, HQ, iii instead -of two as inthe top unit. Thus, modifiedunsymmetrical form of field structure isprovided in which thetop and bottom'units have unequal magnetic fluxdensities. The bottom unit his isalso'prcvid'ed with outer pole elementsor members 123, 125 which connect the 22?, are or" the bar-magnets Hi,its, 12! to the lower ends H35, 1330f the pole pieces'aii, fill. 'As inthe 'ca'seof-the top unit 93], the bar magnets l'i'l, HQ, 1'2! "aredisposed in spaced, parallel relation 'so as to provide air spaces L35,it? between them. The air 'spaces till, 135, 1?? between the bar magnetsin the top and bottom units serve to reduce -resonance in the spacesit'd, l'iiiihetween'the bai'rnagnets and the ends of the ribbon and-pol'epieces.

Also, in our modified form dffieldstructure we have provided pole pieces"95ft? withinterm'ediate sections having concave surface'si'3'9, Ml, thesame as the concave "surfaces 25, '2? "shown in Figs. 1 through 3, forreducing the "sound wave be parallel to the axis of the pole pieces, andthe modified field structure 9i shown in Figs. 5 and 6 shows the axis ofthe bar magnets to be perpendicular to the axis of the pole pieces, thisis merely illustrative. Position of the bar magnets with respect to thepole pieces, their size, and number are not controlling. For thepurposes of our invention, it is necessary, however, that anunsymmetrical magnetic field structure be provided in order to averageout diffraction and resonance effects inherent in symmetricalstructures. While we have illustrated but two forms of unsymmetricalfield structure herein, it is obvious that other combinations andarrangements of bar magnets may be utilized to advantage.

It will readily be seen by those persons skilled in the art, that wehave provided a new type of magnetic and acoustic field structure forthe velocity type microphone which distinctly improves the sensitivityand response thereof. While we have described but two modifications ofour invention, it will be recognized that various other changes andmodifications will, no doubt, readily suggest themselves. We, therefore,desire that the particular form of our invention described herein beconsidered as illustrative and not as limiting.

What is claimed is:

1. A magnetic structure for sound translating apparatus comprising apair of elongated pole pieces disposed in spaced relation to provide anair gap therebetween, each of said pole pieces having a flat facedisposed longitudinally thereof on the side adjacent said air gap, andalso having an intermediate portion of reduced cross section areaforming longitudinal concave surfaces on all sides of said pole pieceexcept on the side containing said fiat face, and a magnetic fieldstructure coupling the ends of said pole pieces to provide a continuousmagnetic flux path between the pole pieces external to said air gap, thepor tion of said magnetic field structure which couples adjacent ends ofsaid pole pieces at one end thereof being larger than the portion ofsaid magnetic field structure which couples the opposite adjacent endsof said pole pieces.

2. A magnetic structure according to claim 1 characterized in that saidmagnetic field structure comprises a pair of magnets each having atleast one magnetizing body, and pole members coupling said pole pieceends to said magnetizing bodies.

3. A magnetic structure according to claim 2 characterized in that eachof said pair of magnets has at least one bar magnet of permanent magnetmaterial, and further characterized in that one of said pair of magnetsis composed of more bar magnets than the other of said pair.

4. A magnetic structure according to claim 1 characterized in that saidmagnetic field structure includes a pair of magnets, each of saidmagnets having a plurality of spaced bar magnets of permanent magnetmaterial.

5. A magnetic structure according to claim 1 characterized in that saidmagnetic field structure comprises a pair of magnets having a pluralityof permanent bar magnets disposed in spaced, parallel relation, each ofsaid bar magnets having its axis disposed parallel to the axis of saidpole pieces, characterized further by the addition of pole membersconnecting an end of said bar magnets to said pole pieces, andcharacterized also by the addition of a plate element connecting theopposite ends of said bar magnets of each of said pair of magnets.

6. A magnetic structure according to claim 1 characterized in that saidmagnetic field structure comprises a pair of magnets, each one of saidpair of magnets having at least one permanent bar magnet the axis ofwhich is disposed perpendicular to the axis of said pole pieces, andcharacterized further by the addition of pole members connecting theends of each of said bar magnets to said ends of said pole pieces.

7. In sound translating apparatus, the combination consisting of (1) apair of elongated pole pieces disposed in spaced, parallel relation toprovide an air gap therebetween, each of said pole pieces having a flatface disposed longitudinally thereof on the side adjacent said air gap,and also having an intermediate portion of reduced cross section areaforming longitudinal concave surfaces on all sides of said pole pieceexcept on the side containing said flat face, (2) a magnetic fieldstructure comprising a pair of magnetizing bodies coupling the ends ofsaid pole pieces to provide a continuous magnetic flux path between saidpole pieces external to said air gap, (3) a ribbon diaphragm mounted insaid air gap for movement in response to diiferences in sound wavepressure on opposite sides thereof, said magnetic field structurecoupling adjacent ends of said pole pieces at one end thereof beinglarger than said magnetic field structure coupling the opposite adjacentends of said pole pieces, and (4) a pair of plates having a plurality ofperforations therein, said plates being mounted in spaced, parallel,overlapping relation to said ribbon diaphragm and forming resonancechambers on opposite sides thereof to provide a standing wave system forsound waves transmitted through said perforations.

8. Sound translating apparatus according to claim 7, wherein each ofsaid magnetizing bodies comprises at least one bar magnet of permanentmagnet material.

9. Sound translating apparatus according to claim 8 wherein each of saidmagnetizing bodies includes a pair of pole members connecting oppositeends of said magnets to adjacent ends of said pole pieces.

10. Sound translating apparatus according to claim '7, wherein saidmagnetizing bodies are disposed with their axes parallel to the axis ofsaid pole pieces.

11. Sound translating apparatus according to claim '1", wherein one ofsaid magnetizing bodies comprises at least two bar magnets of permanentmagnet material, said bar magnets being mounted in spaced, parallelrelation.

12. Sound translating apparatus according to claim 9, wherein saidmagnetizing bodies :are disposed with their axes perpendicular to theaxis of said pole pieces.

HARRY F. OLSON. JOHN PRESTON.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 2,086,834 Williams July 13, 19372,113,364 Anderson Apr. 5, 1938 2,227,530 Harry Jan. 7, 1941 2,346,395Rettinger Apr. 11, 1942 2,348,356 Olson May 9, 1944

